Near Field Communication (NFC) technologies enable contactless communications between devices. The number of applications, along with the corresponding market, for NFC systems grows on a daily basis. The conventional NFC system includes a reader terminal on one side of a contactless communications interface and a plurality of transponders on the other side of the contactless communications interface. An RF identification (RFID) tag is just one example of an NFC transponder device. In many cases, the transponder is a battery-less device that receives power over the contactless communications interface from the reader terminal. Data is communicated from the transponder to the reader terminal using well known passive load modulation (PLM) techniques. In passive load modulation, the transponder harvests some of the field power of the reader terminal and uses that power to modulate the amount of load that the reader terminal antenna senses.
Alternatively, an active transmission of data from the transponder to the reader terminal can be made through the use of active load modulation (ALM). To accomplish active load modulation, the transponder makes an RF transmission that is either in phase with or 180° out of phase with the field of the reader terminal. When the transponder field is in phase with the reader field, there is an increase in voltage at the reader antenna terminals that is sensed by the reader as the transmitted data. Conversely, when the transponder field is 180° out of phase with the reader field, there is a decrease in voltage at the reader antenna terminals that is sensed by the reader as the transmitted data.
In the past few years, a trend has emerged toward enabling NFC functionality on very small devices (such as wearables and mobile phones). The integration of NFC technology into such small devices necessitates the use of an antenna having a relatively speaking small occupied area or volume. As a result, passive load modulation is not feasible. Instead, active load modulation is needed. However, because the transponder antenna is smaller the transponder may not be able to harvest enough energy from the reader field. This is due to insufficient coupling between the transponder antenna and the reader antenna and further because the same antenna is being used in the transponder during transmission.